Use rails secret command in guides

[ryohashimoto]

In Ruby on Rails Guides, it's better to use rails secret instead of rake secret.

[rails-bot]

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[maclover7]

👍 Thank you for your contribution!

[maclover7]

In the future, please add [ci skip] to any commits that involve changing the documentation, to avoid running the entire Rails test suite.

[ryohashimoto]

@maclover7
Thank you. I'll add [ci skip] for changing the documentation.

[maclover7]

thanks! <3

[maclover7]

Partially reverted in #24188.